Probability arguments must be [0, 1], otherwise domain_error
is called.

If the choice of arguments would give a negative scale, domain_error
is called, unless the policy is to ignore, when the negative (impossible)
value of scale is returned.

Find Mean and standard
deviation examples gives simple examples of use of both find_scale
and find_location, and a longer example finding means and standard deviations
of normally distributed weights to meet a specification.